1. Field of the Invention
This invention relates to an optimization control method for a shock absorber having a non-linear kinetic characteristic.
2. Description of the Related Art
Feedback control systems are widely used to maintain the output of a dynamic system at a desired value in spite of external disturbance forces that would move the output away from the desired value. For example, a household furnace controlled by a thermostat is an example of a feedback control system. The thermostat continuously measures the air temperature of the house, and when the temperature falls below a desired minimum temperature, the thermostat turns the furnace on. When the furnace has warmed the air above the desired minimum temperature, then the thermostat turns the furnace off. The thermostat-furnace system maintains the household temperature at a constant value in spite of external disturbances such as a drop in the outside air temperature. Similar types of feedback control are used in many applications.
A central component in a feedback control system is a controlled object, otherwise known as a process "plant," whose output variable is to be controlled. In the above example, the plant is the house, the output variable is the air temperature of the house, and the disturbance is the flow of heat through the walls of the house. The plant is controlled by a control system. In the above example, the control system is the thermostat in combination with the furnace. The thermostat-furnace system uses simple on-off feedback control to maintain the temperature of the house. In many control environments, such as motor shaft position or motor speed control systems, simple on-off feedback control is insufficient. More advanced control systems rely on combinations of proportional feedback control, integral feedback control, and derivative feedback control. Feedback that is the sum of proportional plus integral plus derivative feedback is often referred to as PID control.
The PID control system is a linear control system that is based on a dynamic model of the plant. In classical control systems, a linear dynamic model is obtained in the form of dynamic equations, usually ordinary differential equations. The plant is assumed to be relatively linear, time invariant, and stable. However, many real-world plants are time varying, highly nonlinear, and unstable. For example, the dynamic model may contain parameters (e.g., masses, inductances, aerodynamic coefficients, etc.) which are either poorly known or depend on a changing environment. Under these conditions, a linear PID controller is insufficient.
Evaluating the motion characteristics of a nonlinear plant is often difficult, in part due to the lack of a general analysis method. Conventionally, when controlling a plant with nonlinear motion characteristics, it is common to find certain equilibrium points of the plant and the motion characteristics of the plant are linearized in a vicinity near an equilibrium point. Control is then based on evaluating the pseudo (linearized) motion characteristics near the equilibrium point. This technique works poorly, if at all, for plants described by models that are unstable or dissipative. The optimization control for a non-linear kinetic characteristic of a controlled process has not been well developed. A general analysis method for non-linear kinetic characteristic has not been previously available, so a control device suited for the linear-kinetic characteristic is often substituted. Namely, for the controlled process with the non-linear kinetic characteristic, a suitable balance point for the kinetic characteristic is picked. Then, the kinetic characteristic of the controlled process is linearized in a vicinity of the balance point, whereby the evaluation is conducted relative to pseudo-kinetic characteristics.
However, this method has several disadvantageous. Although the optimization control may be accurately conducted around the balance point, its accuracy decreases beyond this balance point. Further, this method cannot typically keep up with various kinds of environmental changes around the controlled process.
Shock absorbers used for automobiles and motor cycles are one example of a controlled process having the non-linear kinetic characteristic. The optimization of the non-linear kinetic characteristic has been long sought because vehicle's turning performances and ride are greatly affected by the damping characteristic and output of the shock absorbers.